1. Field of the Invention
The present invention relates to an optical communication system or network and an optical node for use therein, and, more particularly, to an optical communication system, typically, of a bus type in which the amplification gain of an optical amplifier in each terminal is controlled in a predetermined manner and an optical node to be used therein.
2. Related Background Art
In recent years attention has been paid to a bus type of optical communication network for its advantageous features. In the bus type of optical communication network, an optical signal transmitted through an optical fiber is transmitted through each terminal connected to the optical fiber without being converted to an electric signal, and in each terminal a means for effecting optical branching and optical combining which is called an optical node is connected to the optical fiber and performs reception and transmission of signals from and to the optical fiber.
When the optical signal is branched in the optical node, the intensity of the optical signal transmitted through the optical fiber is reduced. So, previously the number of terminals that could be connected to the optical fiber was limited, and hence the bus type of optical communication network was believed impractical. However, recently, a means for amplifying the optical signal without converting this to an electric signal, such as a semiconductor optical amplifier and a fiber optical amplifier, has been developed, so it has become possible to increase the number of terminals to be connected to the optical fiber.
Further, while the communication distance of the bus type system cannot be made too long due to the signal delay on a transmission path, the bus type system has an advantage that the terminal connection to the optical fiber can easily be modified. So, the bus type system is suitable for a local area network in which the total transmission distance length is about several kilometers and in which flexibility to re-connect the terminals is desired.
FIGS. 1A and 1B respectively show a schematic structure of such a prior art bus type of an optical communication network and intensities of optical signals on its optical fiber transmission path.
In FIG. 1A, there are provided optical fibers F.sub.1 -F.sub.i+1, terminals T.sub.1 -T.sub.i, optical nodes N.sub.1 -N.sub.i and end equipment S.sub.1 and S.sub.2 for preventing reflections of an optical signal at optical fiber ends. The optical nodes respectively have functions that the electric signals from the terminals T.sub.1 -T.sub.i are converted into optical signals to be transmitted to the optical fibers F.sub.1 -F.sub.i+1, that parts of the optical signals on the optical fibers F.sub.1 -F.sub.i+1 are picked out to be converted into electric signals for transmission to the terminals T.sub.1 -T.sub.i, and that the optical signals on the optical fibers F.sub.1 F.sub.i+1 are amplified to be transmitted to adjacent optical fibers. In FIG. 1B, a line L.sub.1 indicates that an optical signal output from the optical node N.sub.1 is attenuated on the optical fibers F-F.sub.i+1 and that such optical signal is amplified by other optical nodes (i.e., the intensity of the optical signal output from the node N.sub.1) , and a line L.sub.2 indicates the intensity of an optical signal output from the optical node N.sub.3.
In the prior art of FIGS. 1A and 1B, the amplification factors of the optical signal at respective nodes N.sub.1 -N.sub.i are equal to one another.
For example, as shown in FIG. 1B, the signal from the terminal T.sub.1 is converted into the optical signal by the optical node N.sub.1 and output to the optical fibers F.sub.1 and F.sub.2. The optical signal transmitted to the optical fiber F.sub.1 is absorbed by the end equipment S.sub.1. On the other hand, the optical signal transmitted-to the optical fiber F.sub.2 is reduced by an attenuation amount determined by the length of the optical fiber F.sub.2 and enters the optical node N.sub.2. The optical node N.sub.2 picks out part of the incident signal to convert this into an electric signal and transmits this part to the terminal T.sub.2. At the same time, the node N.sub.2 amplifies the remaining part of the incident signal a predetermined gain and transmits this to the optical fiber F.sub.3. This optical signal is transmitted by being repeatedly processed in this manner and reaches the end equipment S.sub.2 after passing through the other optical nodes.
The signal from the terminal T.sub.3 is converted into an electric signal in the optical node N.sub.3 and is transmitted to two adjacent optical fibers F.sub.3 and F.sub.4 to finally reach respective end equipment S.sub.1 and S.sub.2 by the same process as described above.
Although the signals from plural terminals T.sub.1 -T.sub.i cannot simultaneously be transmitted using a common wavelength, practically simultaneous communications can be achieved among plural terminals using a proper access method. Such methods includes a method wherein the signal transmission from a certain terminal is started after the confirmation that no signals are transmitted from the other terminals (called carrier sense multiple access (CSMA)), a method wherein the signal transmission is conducted during a time slot allotted to each individual terminal (called time division multiplexing access (TDMA)), and a method wherein a signal is transmitted from each individual terminal using light whose wavelength is different from those of the other terminals (called wavelength division multiplexing (WDM)).
The prior art system of FIG. 1A, however, has the following drawback.
In this system, the gain value of each node is beforehand determined, as mentioned above. So, if those predetermined gain values are set equal to one another, it would be impossible to freely select the distance between the nodes. In other words, where the distance between nodes is shortened to obtain a multistage connection, intensity of a transmitted optical signal may amount to the saturation level of an optical amplifier used in the optical node. As a result, the signal distortion increases and hence the error rate at the time of transmission may be enhanced.
On the other hand, when the optical communication network is assembled, where the gain of each individual optical node is independently determined so as to prevent the above-discussed saturation, the saturation and shortage of the optical signal may occur in turn if the connection configuration of the optical communication network is altered and hence the distance between the nodes is changed. The reason therefor is that the gain is set to a small value in such an optical node having a short distance to an adjacent optical node while set to a large value in an optical node having a long distance to an adjacent node.
Therefore, the feature of a bus-type optical communication network having a flexible change in the connection configuration is not possible.